High power laser diode bars are commercially available in a variety of configurations including vertical and horizontal stacks such as shown in U.S. Pat. No. 6,272,159. Because the heat generated in a laser bar must be effectively dissipated, it is important that the thermal resistance at the laser bar's interface with the heat sink be low and that an efficient heat sink, such as a microchannel heat sink, constructed of thin copper plates between which water is pumped, be used. A micro-channel heat sink is shown in U.S. Pat. No. 6,268,653. In addition to low thermal resistance at the interface it is important that the materials as the interface have closely related coefficients of thermal expansion to avoid the influence of stress in thermal cycling. The '653 patent suggested that thermal stress from high temperature soldering can be alleviated if the heat sink were composed of a copper-beryllium oxide ceramic-copper sandwich instead of a single material. The laser bar is, in effect, soldered to copper “islands” on one side of a high thermal conductivity ceramic (beryllium oxide, synthetic diamond, or aluminum nitride) which had a coefficient of thermal expansion (CTE) nearer to that of the laser bar. U.S. Pat. No. 5,751,552 discusses the problem of substrate warpage which arises from differences in CTE between the copper strips to which laser components are soldered and the CTE of the ceramic substrate. Warpage of the substrate reduces the surface area available for heat transfer. The '552 patent proposed that conductive material be cast on the back surface of the ceramic substrate to counteract such warpage. Despite such approaches, premature failures still occur at high power outputs, especially in laser bars subjected to pulsed operation where alternate heating and cooling can shorten the useful life (generally defined as a 20% reduction in useful output power) from 10,000 hours to only 1000 hours. It would be extremely advantageous to be able to directly solder a laser bar to the copper surface of a conventional microchannel heat sink without inducing destructive thermal stresses and without requiring a special substrate.